The invention disclosed herein relates generally to permanent magnet motors, and more particularly to a simplified stepper motor housing and pole structure design which assures good magnetic circuit characteristics with a minimum number of simple, easily assembled parts.
Stepper motors are widely used in disc drives, printers, and other computer peripheral equipment, word processing typewriters and actuators for a variety of other applications. Increasingly higher speeds are being required from such equipment, which necessitates greater motor torque. In addition, increasingly precise performance is being required, which necessitates greater motor stepping accuracy. Concurrently, there is a continuing requirement to reduce cost. Thus, it is important to minimize parts count, simplify parts design, reduce possibilities for assembly errors and simplify required tooling and manufacturing steps. Along with the foregoing there are demands for size reduction.
A stepper motor design which has shown potential for improved performance, cost and size characteristics is one in which a permanent magnet rotor is caged within a plurality of poles comprising projections on inner and outer stator members which are stacked together with the outer stator members being cup shaped and serving as a housing for electrical windings. The inner stator members are disc shaped and connected back to back, each with a central opening surrounded by poles projecting in opposite directions. The outer cup shaped members also each have a central opening surrounded by inwardly projecting poles interleaved with the outwardly projecting poles on the inner stator members. An electrical winding is located within each outer stator member outside the poles. A stepper motor of this design is disclosed in U.S. Pat. No. 4,355,248 issued to H. Manson on Oct. 19, 1982.
Motor torque is affected by the characteristics of the magnetic material from which the rotor is made, the number of turns of wire used in the windings and reluctance of the magnetic circuits, which is increased by air gaps and constrictions in magnetic materials in the magnetic flux paths. Higher performance magnetic materials are generally higher in cost, thus requiring a tradeoff between cost and performance. More turns of wire in the windings requires more space, which increases the motor size. With reference to minimizing reluctance of the magnetic circuits and to providing stepping accuracy, both require maintaining high dimensional accuracy during fabrication and rigid positioning of stator elements.
The design disclosed in U.S. Pat. No. 4,355,248 provides improvements in the areas of insuring pole alignment accuracy and providing rigid connections between the inner and outer stator members. This is accomplished by forming the inner stator members and assembling them back to back so that their peripheral edges are slightly separated. The outer cup shaped stator members are sized to fit over the edges of the inner stator members and are staked along the edges at several points around the circumference so as to be deformed into the separation between the inner stator members. A separate mounting flange is attached to one side of one of the outer stator members to serve as means for mounting the motor.
Some disadvantages of this design are in the areas of parts count and simplicity, ease of assembly and magnetic circuit reluctance. In regard to the latter area, a potential location of relatively high magnetic reluctance occurs at the junction of the inner and outer stator members. Each outer stator member contacts an inner stator member only along the edge of the inner member. The outer member is formed by a drawing operation, and, thus, is prone to certain variations and inaccuracies as indicated in the previously identified patent. Accordingly, firm contact between the inner and outer members is assured only at the points at which the outer member is staked. Elsewhere there may be air gaps or limited contact between the members.
The process by which the inner stator members are attached back to back is described as involving welding or a suitable adhesive. During this process the members are apparently maintained in proper alignment by a jig or fixture using alignment holes in the members. Thus, an alignment fixture, welding or bonding equipment and staking equipment are required for motor assembly.
The applicant has avoided the above noted disadvantages by providing a design comprising a minimum number of simple self aligning parts requiring no special assembly equipment. The parts assembly is not subject to critical tolerance areas, and provides firm contact between parts. Thus, low reluctance magnetic paths between parts are assured.